A known technique of increasing information throughput in synchronous signaling systems that communicate digital information is to use increased bit rates to communicate the information. Older wide area radio communication systems, such as systems using the POCSAG (Post Office Commission Standards Advisory Group) protocol, were designed to operate using two level frequency shift modulation at one bit rate for all systems, all the time. Newer systems, such as the well known FLEX.TM. family of signaling protocols licensed by Motorola, Inc., of Schaumburg, Ill., allow the use of bit rates selected from a set of predetermined bit rates. In the FLEX family of protocols, multilevel frequency shift modulation in the specific form of four level frequency shift modulation (hereafter referred to simply as four level modulation) is used for the highest bit rate (6400 bits per second (bps)), and can be used for 3200 bps.
Referring to FIG. 1, a timing diagram is shown which illustrates features of the transmission format of the well known FLEX signaling protocol. The signaling protocol comprises a FLEX frame 105 that has a duration of 1.875 seconds. The FLEX frame 105 includes a synchronization (SYNC) portion 106 and eleven blocks 125, identified as blocks zero through ten. The SYNC portion 106 includes a preamble portion 110 and a SYNC 2 portion 120. The preamble portion 110 includes a SYNC 1 portion and a Frame Information Word. Synchronization and certain global information are transmitted in the preamble portion 110 using a preamble of 1600 bits per second (bps) with a two level frequency shift modulation, in which deviations of .+-.4800 Hertz are preferably used for the two frequency shift levels. The global information indicates to all receivers that receive the frame a bit rate and level of modulation at which the remainder of the frame 105 will be transmitted. The remainder of the frame comprises the SYNC 2 120 and the eleven blocks 125. A boundary 115 between the preamble portion 110 and the remainder of the frame is at the end of the preamble portion 110. The remainder of the frame can be transmitted using either two or four level frequency shift modulation at 1600 or 3200 symbols per second, resulting in bit rates of 1600, 3200 or 6400 bps.
A known technique used in a radio receiver for improving the sensitivity of receiving an FM digital radio signal sent at an actual carrier frequency that is offset from a nominal carrier frequency is to measure the offset in the receiver and use it to eliminate the offset. The nominal carrier frequency as used herein means the receive carrier frequency as referenced to the actual receiver injection frequency in the case of a conversion receiver, or the ideal transmitted carrier frequency in the event the receiver is a direct carrier demodulation receiver. In a digital signal that is frequency shift modulated, a technique commonly used to measure the offset is to measure the average frequency deviations of the frequency peaks and valleys and determine from them an average deviation, which essentially is the offset. When this technique for eliminating offset is used in a receiver having an intermediate frequency (IF) filter substantially optimized for receiving two level modulation in an system in which two level modulation is used and the modulation index (MI) is reasonably optimized (i.e., the MI is on the order of 1.0), an improvement of receiving sensitivity can be achieved over a range of offset amounts.
Referring to FIG. 2, a graph is shown which has two curves showing estimated receiving sensitivities of a exemplary prior art radio that is designed to recover such a digital signal, for which the MI and the bandwidth of the receiver IF are reasonably optimized. Bit error rate is plotted, and represents the signaling sensitivity. Curve 205 illustrates the bit error rate versus the amount of frequency offset between the actual carrier frequency of the received digital signal and the nominal carrier frequency, without offset correction being used, for digital signals having random data. Curve 210 illustrates the bit error rate versus frequency offset f or the same digital signals when frequency offset is used. It will be appreciated that the use of frequency offset correction provides substantial bit error rate improvement compared to not using frequency correction.
The situation can be different in a receiver that has an IF filter that is optimized for multilevel signaling, such as the four level signaling used in the FLEX family of protocols. In such a receiver, the use of frequency offset correction can cause a degradation of bit error rate (and, therefore also the signaling sensitivity) when a two level signal is received in which the MI is not reasonably optimal.
Thus, what is needed is a technique for providing improved signaling sensitivity in a receiver designed to receive signals having multiple levels of frequency shift modulation, such as two and four level modulation.